What a creative idea and you executed it quite nicely, it seems.

I'm a little unclear as to the purpose, I guess, because if you have a stage piano, isn't the point of it to be able to move it around easily? But for those wanting a piano "look" but a digital keyboard, this is a great idea. I'm not sure how many people are out there in this category. I'm also not sure what I would price it, but that really ought to be based on what you spend making them - time, effort, and materials. You know, honestly, there are so many free uprights out there, I wonder if it would be possible to start with a real piano and take the "guts" out to accommodate a keyboard? hmmmm....
A couple of suggestions... Is there room in the empty "upright" for a good set of speakers? Then you could cover the back (or maybe even the front - see examples in some older uprights) with mesh and have some better sound options that are all contained within the unit. Also, if it were me, I'd use piano hinges, not door hinges. That will help the look a lot, in my opinion.
Love your thought process and creativity!

JH